Certain types of infrared detectors are designed to operate at extremely low temperatures. For this purpose, focal plane arrays consisting of many individual sensors are typically mounted on the closed end of a glass conduit into which liquid nitrogen, cryostats or cryocooler systems are introduced. Because frost formation on the detector tube's window would destroy the detector's ability to see, and to provide the required cryogenic temperature at the detector array with ambient temperature electrical interconnects to the outside world, it is necessary to thermally insulate the cooled array from the outside world by enclosing the detector in a dewar at a high vacuum. The window through which infrared radiation impinges on the sensor array is part of the envelope of the dewar, and the cooled portion of the dewar consists of the detector-bearing cryogenic fluid conduit or stem which protrudes into the dewar's envelope opposite the window. A critical problem area with all dewars is the method used to route the electrical connections across the temperature gradient between the detector and the outside world, and through the evacuated envelope.
Because each sensor of the sensor array needs to be separately connected to the outside world, it is necessary to bring a very large number of leads (on the order of fifty to a hundred) out from the dewar in a very small space. Typically, the stem, on which the electrical conductors or leads are usually mounted, has a diameter of less than one centimeter and a length on the order of six to seven centimeters.
Where the conductors have to go through the glass envelope of the dewar, the use of Kovar as a conductor material is indicated. The reason for this is that Kovar readily forms an oxide coating which bonds firmly to glass, and is used as the interconnect material because of its matched thermal expansion to that of glass and because of its relatively low heat load across the thermal interface. The latter property makes it desirable to use Kovar for the entire lead run from the detector plane to the base of the dewar because the exclusive use of Kovar substantially reduces the heat leak along the leads, as compared to prior art devices. Use of a Kovar leadframe configuration permits the number, spacing and cross section of individual leads to be tailored to the application.
Kovar leadframes of the size encountered in the abovediscussed type of infrared dewars warp or deform easily and oxidize severely in the presence of air while being bonded to glass at high temperatures. In a production environment, it has long been considered impractical to produce leadframes longer than two or three centimeters at the most. Consequently, it has been the general practice throughout the industry in the prior art to use Kovar leads only for the penetration of the glass wall of the dewar envelope. The stem surface inside the envelope was provided with conductors of another type, which were applied in a separate operation and were individually connected to the Kovar leads. For example, in one technique, the stem surface inside the envelope was coated with a vacuum-deposited layer of noble metal, and laser scribing techniques were used to transform the continuous noble metal surface into individual leads.
The problem with the prior art approach was that it was not only delicate, cumbersome, expensive, and subject to excessive heat leaks, but that it also required two junctions for each lead within the envelope (one junction between the sensor's lead wire and the noble metal lead, and another between the noble metal lead and the Kovar lead), thereby reducing the reliability of the finished product. The reliability of the prior art product was further reduced because the laser-scribed unreinforced portion of the stem was more vulnerable to breakage.